Flow cytometers typically analyze particles by applying a stain or reagent, suspending them within a stream of sheath fluid, and passing the stream linearly through a laser. When the laser strikes the particles, it produces several channels of information in the form of scattered light and emitted light at different wavelengths. Traditionally photo diodes and/or photo multiplier tubes are then used to convert the optical signal into an electronic signal, which may then be measured and characterized. However, depending on particle size, the particular chemicals used, laser power, and many other factors, the signals produced may occupy a large range of magnitudes. A small, unstained particle or “negative” particle may produce a signal with magnitude of only 1/1000th or less than that of a large, stained particle. Furthermore, some systems require analyzing particles at rates of over 50,000 per second, but also possibly as slow as only several per second. Accurate analysis over this large range of magnitudes and event rates has introduced many problems.
Early systems designed to analyze these signals used an analog approach for peak determination of the linear, logarithmic, and integral values during a gated window, and then digitized the peak values at the end of the window. These systems sometimes took an input signal, used a standard base line restoration (BLR) circuit to remove DC offsets, then used a linear amplifier to create the linear value, a logarithmic amplifier to create the logarithmic value, and an integrator circuit to calculate the integral during the window. An analog peak sample and hold circuit (PSH) was sometimes used to find the maximum value during the event, and this maximum value was sometimes then converted with an analog to digital converter (ADC) to get a digital value. The logarithmic amplifier was sometimes used to allow a wider dynamic range of analysis than the linear signal could provide. These types of systems give a good starting point for signal analysis, but have some inherent problems. Some of these problems include:                1. Analog logarithmic amplifiers usually do not have a perfect transfer function and can contain “ripples” that may distort the input signal. Thus, the output can be higher or lower than it should have been depending upon the input magnitude, which introduced errors into calculations and distorted population histograms.        2. Analog PSH circuits can have several problems, such as noise susceptibility, that may add to the inherent error of the system.        3. In some case, especially at very high event rates, the standard BLR circuit may introduce errors of its own. This error, which may manifest itself as a DC bias, may cause pulse distortion as the logarithmic amplifier input crosses from positive to negative and back to positive.        
Perhaps to eliminate some of these problems, a second system was developed that eliminated the logarithmic amplifier. In some arrangements this system took the input signal, applied different levels of linear gain, used gated analog PSH, used comparators to determine which range to use, and used a processor to shift the data to the appropriate place. The multiple linear gain stages were sometimes necessary to provide adequate dynamic range.
The second system may sometimes have removed the non-linearity problem of the logarithmic amplifier by not using one. However, these systems did not always eliminate all of the problems, and sometimes introduced some of their own. Some of the problems with this system include:                1. The same analog PSH problems noted above.        2. Transitions from one gain stage to another may not be necessarily perfect, and may create “elbow” effects of non-linear regions.        
Perhaps in an attempt to eliminate some of the original problems in a similar manner, a third system was created. In some arrangements, this system may have taken the input signal, may have used ADCs to directly sample the signal, then sometimes may have used a processor to perform digital PSH functionality. A high resolution ADC was sometimes used to achieve the desired dynamic range.
This third system may have removed the non-linearity problem of the logarithmic amplifier, the problems of the analog PSH and the problems of using multiple gain stages. However, this type of system also may have introduced some problems.
The problems with this type of system include analysis dynamic range is sometimes limited to ADC resolution perhaps because the signal is directly sampled. With this system, very high resolution sometimes is required to detect and characterize signals of low magnitude. For example, using a normalized maximum input of 10V to the ADC, to get a fairly standard 12 bits of resolution over 4 decades (80 dB), the ADC may need to have at least 1024 steps in the last decade. In situations where the last decade is a maximum of 10 mV, each step may need to be 10 μV, which may require an ADC with at least 20 bits. Using an ADC with lower resolution could result in quantized, or “picket fence,” results. To achieve 5 decades (100 dB), the ADC may need at least 24 bits, and 6 decades (120 dB) may require 27 bits. This may mean that system dynamic range may be limited by ADC technology. To achieve the necessary high resolution, sample speed may need to be given up, which may limit the speed that the signal may be sampled at, which may further limit the speed of overall analysis. A trade-off between analysis rates and resolution then may need to be made, which may make the lower end of the dynamic range quantized.
Prior work for particle analysis electronics in flow cytometry has apparently failed to solve these challenges without introducing new problems. Consequently, there is still a need for analysis electronics that can provide high dynamic range analysis at high event rates.